Because,
कायेन मनसा बुद्ध्या केवलैरिन्द्रियैरपि ।
योगिनः कर्म कुर्वन्ति सङ्गं त्यक्त्वाऽऽत्मशुद्धये ॥ ११ ॥
kāyena manasā buddhyā kevalairindriyairapi .
yoginaḥ karma kurvanti saṅgaṃ tyaktvā’’tmaśuddhaye .. 11 ..
By the body, by the mind, by the intellect, by mere senses also, Yogins perform action, without attachment, for the purification of the self.
Read the commentary
Mere : free from egotism, resolute in the faith that ‘I act only for the sake of the Lord, not for my benefit.’ ‘Mere’ should be construed along with ‘body’ &c., with each one of them separately, Yogins are those who are devoted to works, free from egotism in all their acts, without attachment for their results. They act only for the purification of the mind (sattva). Wherefore, as thy duty lies only there, do thou only perform action.